Diazo resins prepared by reacting an aromatic diazonium salt, for example, diphenylamino-4-diazonium salt with an active carbonyl compound, for example, formaldehyde have widely been applied, particularly in the printing field as a photosensitive material for preparing an offset printing plate by taking advantage of the change of their solubility upon photolysis.
An offset printing plate is made by mixing the photosensitive material such as the diazo resin with a binder and the like followed by coating on a substrate of hydrophilic metal or paper, preferably aluninum. Upon imagewise exposure, the exposed areas in the printing plate is cured to become insoluble to a developer. Thus, by treating with the developer hydrophilic and oleophilic areas are formed on the plate, enabling to print with water and fatty ink.
Recently, it has been highly demanded in view of the workability or the like to increase the sensitivity of the printing plate. To this end, the development of the photosensitive material is proceeding.
It has been known that the photosensitive material comprising a mixture of the diazo compound with a high molecular weight compound is improved as compared with the photosensitive material comprising the diazo compound only because it has no requirement of subsequent lacquor application and exhibits an excellent performance of giving a stable printing plate. However, if the high molecular weight compound in the photosensitive composition is hydrophilic, the storage stability and the ink receptivity are poor to result in practical problems, although the developability and the compatability with the diazo resin are excellent. While on the other hand, if the high molecular weight compound is oleophilic, the developability, particularly with an aqueous developer is poor and unsatisfactory although the storage stability and the ink receptivity are excellent. Thus, the selection of the high molecular weight compound is a significant problem.
In view of the above, various kinds of high molecular weight compounds have been proposed. For example, in Japanese Patent Application Laid-Open No. 30604/1975, a polymer having the structural unit of the following general formula (II) was disclosed, but it is not still satisfactory due to its poor ink receptivity: ##STR2## wherein R.sup.4 represents a hydrogen atom or a methyl group, R.sup.5 represents a hydrogen atom or a methyl, ethyl or chloromethyl group and m represents an integer from 1 to 10.
Further, in Japanese Patent Application Laid-Open No. 4144/1981 an acrylate polymer without a hydroxyl group-containing monomer was disclosed. This polymer is more excellent than the former polymer in the ink receptivity, but it is not satisfactory in the photosensitivity and the coatability. In Japanese Patent Application Laid-Open No. 98614/1979 is disclosed a high molecular compound comprising as an essential ingredient an aromatic hydroxyl group-containing monomer. Although this compound has a well balanced performance in view of the photosensitivity, the developability, the ink receptivity and the coatability, the stability under severe storage conditions, for example, at 80% humidity and 40.degree. C. temperature, etc. is not acceptable.
While, the diazo resins have been also examined since they give a significant effect on the printing plate as in the high molecular weight compound. For example, Japanese Patent Publication No. 1167/1972 and U.S. Patent Specification No. 3300309 disclose organic salts such as organic sulfonates as an anion of diazo group, but they are inconvenient in use since the image upon exposure is visualizable with difficulty. In Japanese Patent Application Laid-Open No. 98613/1979 is disclosed inorganic salts such as halogenated Lewis acid salts as the anion of diazo group. They are unstable under severe storage conditions such as high temperature and high humidity, although the satisfactory visible images upon exposure can be obtained.
As described above, the high molecular weight compound as well as the diazo resin have been widely examined. However, it is difficult to obtain the satisfactory developability and storage stability even if they are combined. For solving the foregoing defects, the addition of an acid such as oxalic acid or tartaric acid (Japanese Patent Application Laid-Open Nos. 3216/1978 and 65430/1983), the addition of a high molecular weight organic acid (Japanese Patent Application Laid-Open No. 107238/1981) or the application of a subbing layer (Japanese Patent Application Laid-Open No. 21126/1981) has also been known, but any of them is unsuccessful.
An object of this invention is to provide a photosensitive composition excellent in the storage stability and the developability.
Another object of this invention is to provide a photosensitive composition having high sensitivity and being free from the contaminations in the background areas.
A further object of this invention is to provide a photosensitive layer having the film strength enough to withstand the overdevelopment and the print.
These and other objects of this invention will become apparent from the following description.